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Gegevens analyse
Les 1
Fedor Wagenaar
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Data in Tables
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The Key Characteristic of
Databases: Related Tables
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Database Examples
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MS Access in Detail
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Enterprise-Class Database
Systems
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The Database
A database is a self-describing collectionof integrated tables
The tables are called integrated becausethey store data about the relationshipsbetween the rows of data
A database is called self-describing
because it stores a description of itself The self-describing data are called
metadata, which is data about data
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Typical Metadata Tables
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Database Contents
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Three Types ofDatabase Design
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Database Design from Existing Data
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Data Import: One or Two Tables?
This is an important
decision, and based
on a set of rules
known as
normalization (whichis covered in Chapter
Three)
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Database Design from New
Systems
Development
Entity-Relationship data modeling is covered in Chapter Five, and data model
transformations to database designs are covered in ChapterSix
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Database Design from Database
Redesign
Database redesign is
covered in Chapter
Eight, after coverage of
SQL in ChapterSeven
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Knowledge Priorities
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The Data Model
A data model is a plan, or blueprint, for a
database design.
A data model is more generalized andabstract than a database design.
It is easier to change a data model than it
is to change a database design, so it is the
appropriate place to work through
conceptual database problems.
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Entities
Something that can be identified and theusers want to track
Entity class a collection of entities of a
given type Entity instance the occurence of a
particular entity
There are usually many instances of anentity in an entity class.
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CUSTOMER:The Entity Class and Two Entity Instances
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Attributes
Attributes describe an entityscharacteristics.
All entity instances of a given entity classhave the same attributes, but vary in thevalues of those attributes.
Originally shown in data models as
ellipses. Data modeling products today commonly
show attributes in rectangular form.
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EMPLOYEE:
Attributes in Ellipses
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EMPLOYEE:
Attributes in Entity Rectangle
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Identifiers
Identifiers are attributes that name, or identify, entity
instances.
The identifier of an entity instance consists of one or
more of the entitys attributes. Composite identifiers: Identifiers that consist of two or
more attributes
Identifiers in data models become keys in database
designs:
Entities have identifiers.
Tables (or relations) have keys.
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Entity Attribute Display
in Data Models
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Relationships
Entities can be associated with one another in
relationships: Relationship classes: associations among entity classes
Relationship instances: associations among entity instances
In the original E-R model, relationships could
have attributes but today this is no longer done.
A relationship class can involve two or more
entity classes.
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Degree of the Relationship
The degree of the relationship is the number of
entity classes in the relationship:
Two entities have a binary relationship of degree
two. Three entities have a ternary relationship of degree
three.
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Binary Relationship
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Ternary Relationship
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Entities and Tables
The principle difference between an entity
and a table (relation) is that you can
express a relationship between entities
without using foreign keys.
This makes it easier to work with entities in
the early design process where the very
existence of entities and the relationshipsbetween them is uncertain.
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Cardinality
Cardinality means count, and is
expressed as a number.
Maximum cardinality is the maximumnumber of entity instances that can
participate in a relationship.
Minimum cardinality is the minimum
number of entity instances that must
participate in a relationship.
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Maximum Cardinality
Maximum cardinality is the maximum
number of entity instances that can
participate in a relationship.
There are three types of maximum
cardinality:
One-to-One [1:1]
One-to-Many [1:N]
Many-to-Many [N:M]
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The Three Types of
Maximum Cardinality
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Parent and Child Entities
In a one-to-many relationship: The entity on the one side of the relationship is called
the parent entity or just the parent.
The entity on the many side of the relationship iscalled the child entity or just the child.
In the figure below, EMPLOYEE is the parentand COMPUTER is the child:
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Minimum Cardinality
Minimum cardinality is the minimum number ofentity instances that must participate in arelationship.
Minimums are generally stated as either zero orone: IF zero [0] THEN participation in the relationship by
the entity is optional, and no entity instance mustparticipate in the relationship.
IF one [1] THEN participation in the relationship bythe entity is mandatory, and at least one entityinstance must participate in the relationship.
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Indicating Minimum Cardinality
As shown in the examples in a following
slide:
Minimum cardinality of zero [0] indicating
optional participation is indicated by placing
an oval next to the optional entity.
Minimum cardinality of one [1] indicating
mandatory (required) participation isindicated by placing a vertical hash mark
next to the required entity.
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Reading Minimum Cardinality
Look toward the entity in question:
IF you see an oval THEN that entity is
optional (minimum cardinality of zero [0]).
IF you see a vertical hash mark THEN that
entity is mandatory (required) (minimum
cardinality of one [1]).
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The Three Types of
Minimum Cardinality
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Data Modeling Notation
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Data Modeling Notation:
ERwin
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Data Modeling Notation:
N:M and O-MNote that:
(1) ERwin cannotindicate trueminimum
cardinalities onN:M relationships
(2) Visio introducesthe intersectiontable instead of
using a true N:Mmodel